1. Field of the Invention
The present invention relates to a camera, and more particularly to a single lens reflex type camera.
2. Description of the Prior Art
Almost any single reflex camera is provided with a quick return mechanism for a mirror associated with a finder. Such quick return mechanism comprises a quick return setting member, a mirror-up spring and an associated return spring, a setting pawl, and a mirror-up member. A shutter charging operation energizes or stresses both the mirror-up spring and the return spring while bringing the quick return setting member to its set position where it is locked by the setting pawl. A shutter release causes the mirror-up member to be initially actuated under the resilience of the mirror-up spring to drive the mirror upward, followed by unlocking the quick return setting member from the setting pawl immediately before the termination of shutter operation, allowing the setting member to be returned under the resilience of the spring until the mirror returns to its original position.
Unlocking of the setting member from the setting pawl takes place immediately before the termination of a shutter operation with the aid of the force of a focal plane shutter, for example. However, the magnitude of such force is low, and therefore it is necessary that the setting member, which is biased by the return spring, can be unlocked under as small a force as possible. This presented a limit on the magnitude of the resilience of the return spring.
Momentarily referring to FIG. 19, it is necessary that the resilience of the return spring be greater than a loading on the setting member, indicated in phantom line, over the full extent of movement. In terms of efficiency, the resilience of the return spring may slightly exceed the loading, as indicated by a solid line in FIG. 19. However, when the return spring is stressed, its resilience increases beyond a required value, as indicated by broken lines in FIG. 19, thus causing the setting member to apply an increased force to the setting pawl, which must then be unlocked under a greater force. As mentioned previously, the force used to unlock the setting pawl is limited.
As a result, in a conventional quick return mechanism for a single lens reflex camera, there is a critical balance between the resilience of the return spring and the magnitude of the force which is required to unlock the setting member, requiring a delicate adjustment during assembly.
In addition to a main mirror for the finder, certain single lens reflex cameras include a sub-mirror in order to introduce light passing through a portion of the main mirror onto a light receiving element used for exposure control or onto a light receiving element used for distance determination. In certain arrangements, a mirror of an increased size is used to prevent a vignetting effect of the field of view of the finder, by swinging the main mirror back during the mirror-up mode (see Japanese Laid-open Patent Application No. 144,122/1977). However, a conventional mirror drive mechanism for single lens reflex camera which is provided with a sub-mirror and a main mirror arranged to be swung back as mentioned above requires a bias swing force supplied from a spring, and hence involves an increased amount of loss of force. A problem then arises that an increased force is required to drive the mirror. In addition, the operating condition and the accuracy achieved of the mirror can be checked only after the entire mechanism is assembled into a mirror box, causing inconvenience in assembly and adjustment.
In a conventional single reflex camera, it is common to position a quick return mechanism for the mirror of the finder on the bottom of and inside a mirror box. However, the recent trend is toward the provision of a light receiving element internally within the bottom of the mirror box so that such element determines light passing through the main mirror and reflected by the sub-mirror to control the exposure process or to determine the distance. This prevents the disposition of the quick return mechanism within the bottom of the mirror box. Accordingly, such quick return mechanism is disposed outside the bottom of the mirror box in certain arrangements. However, at any rate, the quick return mechanism must be charged by stressing both the mirror-up spring and the return spring, and this causes a force of a relatively high magnitude to be applied to the mirror box in the lateral direction, resulting in the drawback that the mirror box may be deformed, in particular, when the mirror box is assembled from pressed parts. This resulted in changing the position of the finder mirror to cause a defocusing of the finder or in causing a defocusing of the surface of the light receiving element to prevent an accurate distance determination being achieved if such element is positioned within the mirror box.
In certain single lens reflex cameras, incident light passing through a receiving lens is caused to impinge on a light receiving element which is used for exposure control or for distance determination, by causing part of such incident light to be reflected by a sub-mirror, located rearwardly of the main mirror, after such light has passed through the main mirror. It is required that the light receiving surface of such element does not exhibit any tilting with respect to the optical axis of incidence to avoid partial blurring in order for an accurate photometry or distance measurement to be performed. The tilting of the light receiving surface of such element may occur with respect to the optical axis of incidence in either the left-and-right direction or the fore-and-aft direction, as viewed from the front side of the camera. In the prior art practice, such tilting is adjusted by the use of adjusting screws which abut against the light receiving element at three points.
While such adjusting mechanism of the prior art is simple in construction, the actual adjustment is greatly troublesome in that adjustment of the tilting in the left-and-right direction, for example, causes a change in the tilting in the fore-and-aft direction and vice versa. For focusing adjustment of the light receiving element which is used to determine the distance, the three adjusting screws must be turned simultaneously an equal amount, which involves great difficulty in avoiding any resulting tilting. If optical means such as collimator is used to perform such adjustment, the tilting in the left-and-right and the fore-and-aft direction can be evaluated simultaneously, and hence the degree of difficulty of adjustment can be lessened. However, for the light receiving element which is used to determine the distance, the electrical evaluation is employed. Since the evaluation of the tilting in the left-and-right direction and the evaluation of the tilting in the fore-and-aft direction takes place separately, it is necessary to switch electrical outputs for evaluation in order to confirm any change in the tilting in one direction which results from the adjustment in the other direction, thus resulting in a very troublesome operation.
In a single lens reflex camera of internal photometry type in which light passing through a taking lens is utilized to derive a distance signal, it is desirable that the light receiving element which is used to determine the distance undergoes a variety of adjustments during different stages of assembly of the camera. For example, an electrical adjustment is initially required to determine the compatability of the light receiving element with an arithmetic unit associated therewith. Subsequently when the element is assembled into the mirror box, tilting of the element is adjusted to remove a partial blurring. Later when the mirror box is assembled within the camera, the element is caused to move fore-and-aft along the optical axis in order to achieve a focusing adjustment of the element.
However, in the conventional single lens reflex cameras, it is difficult to derive an electrical signal from the light receiving element out of the camera once such element or the mirror box in which it is disposed is assembled within the camera. No proper adjustment can be made unless the light receiving element is assembled within the mirror box. This evaluation is enabled only after the mirror box has been assembled. It thus becomes difficult to perform the adjustment since such takes place on the basis of such evaluation. On the other hand, if the element fails to exhibit a desired characteristic after it has been assembled and hence must be changed, replacement of the element and a processing circuit to which it is directly fixed is not a simple matter to perform.